Process for the determination of MTBE in the ground and air

ABSTRACT

A process is described, and also the system for its embodiment, for the determination of pollution by MTBE in the soil and in the overlying atmosphere. An example is described relating to the monitoring of underground fuel tanks for autotraction containing an oxygenated additive.

The present invention relates to a process for the determination ofpollution by methyl ter butyl ether.

Methyl ter butyl ether (MTBE) is the most widely used among oxygenatedadditives for motor vehicles. Its addition improves combustion andsignificantly reduces the emission of carbon monoxide, especially duringlow winter temperatures. The possibility of a leakage in the earth offuels contained in underground tanks of service stations is probable. Asa result of this, MTBE has been the object of a great deal of researchwith respect to its destiny in the environment and its potential impacton public health, mainly bearing in mind that this substance isextremely volatile and soluble in water. In addition, if present, itremains in deep water and sediments as owing to its very limitedbiodegradability, with an odour that can be noticed starting fromconcentrations at a level of 20 ppb. Its cancerogenous activity, ifexisting, seems to be small.

There are various methods for determining and measuring MTBE: they rangefrom gas chromatography to IRA and flame-ionization, but they are alldifficult to apply to the ground.

We have now overcome these problems by means of a process which allowsthe continuous monitoring of MTBE, in the ground and on the surface,using sensors in the solid state.

In accordance with this, the present invention relates to a process formonitoring methyl ter butyl ether (MTBE) vapours, in concentrationsequal to or higher than 0.1 ppm, in the ground and overlying atmospherecomprising:

-   a) adopting a series of MTBE vapour sensors of which at least one in    the earth, equipped with a membrane permeable to gases and    impermeable to water, and at least one in the air on the surface of    the ground, these sensors consisting of-   a sensitive element made of a semi-conductor metal oxide containing    platinum;-   a heater capable of bringing the temperature of said sensitive    element to a range of 300 and 500° C.;-   b) continuously observing the resistance variations of the sensitive    elements by interaction with MTBE,-   comparing the signals emitted by the sensor in the earth and the    sensor in the air on the ground-surface;-   evaluating on the basis of this comparison the presence and    concentration of MTBE in the surface layers or depths of the ground    and in the atmosphere above the ground itself.

A further object of the present invention relates to the device foreffecting the process.

A typical embodiment of the invention is described hereunder, withreference to FIGS. 1 and 2 in which equal numbers correspond to equalelements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a sensor according to one embodiment of theinvention;

FIG. 2 illustrates an underground tank according to an embodiment of theinvention;

FIG. 3(a) illustrates the relationship between the kinetics response ofa sensor according to one embodiment of the invention and time atvarious concentrations of gasoline;

FIG. 3(b) illustrates the relationship between the kinetics response ofthe sensor and the concentration of gasoline;

FIGS. 4(a) and 4(b) illustrate the kinetics response of a systemincluding two MTBE sensors and electronic control unit according to anembodiment of the invention.

FIG. 1 illustrates a sensor, in enlarged form. The sensitive element 1is produced by placing by screen printing, on an aluminum slab withdimensions of 3×9×0.25 mm, a 40 micron layer of a tin oxide paste,containing platinum. Powders are used having a particle size of lessthan 1 micron containing from 20 to 30% by weight of alumina andorganometallic platinum as catalyst in a quantity ranging from 0.1 to 1%by weight. A resistor (consisting of a layer of any commercial screenprinting conductor paste capable of resisting at least 400° C.) isdeposited, again by screen printing, on the opposite side of the slab,to keep the sensitive element at an operating temperature of 300-500° C.After depositing the electric contacts also by screen printing, the slabis subjected to a baking step in an oven at 800-1000° C. for an hour.

Finally the device, which forms the sensitive element, is assembled on aT078 2 container and inserted in a steel cylinder 3 closed by means of aflame-shield net 4. If the sensor described is fixed into the ground, amembrane 5, permeable to gases and impermeable to water, is insertedunder the flame-shield net to prevent any possible water present in theearth from entering into contact with the sensitive element. Anappropriate porous septum or even better a membrane made of ePFTEmaterial can be used for the purpose.

The sensitive elements can alternatively be produced with other types ofsemi-conductor metal oxides, but still using platinum as catalyst.

The sensors are equipped with feeders, or alternatively batteries, tosupply energy to the heater and resistivity measurement circuit of thesensitive element.

FIG. 2 illustrates an underground tank 6 of a service station forleadless fuels with a configuration with three sensors for theembodiment of the present invention. Two sensors 7, like those describedwith a gas permeable membrane, are fixed in the ground at the sides ofthe tank, a sensor 8 without a membrane inserted in the chamber 9 abovethe tank. 10 illustrates the data acquisition switchboard.

Sensors such as those described above have a sensitivity which is suchas to signal the presence of vapours of gasoline containing MTBE or MTBEalone with concentrations even less than 1 ppm in the air. Thepossibility of comparing the signals coming from the sensors fixed inthe ground with those situated in the chamber above the tank over aperiod of time, make it possible to distinguish between leakages on theground surface and losses from the underground tank.

In another embodiment of the same invention, sensors can be placed alongan underground pipe around it and on the ground surface above. In thiscase the signals emitted from the sensors can be sent via radio to acentral unit for collection and processing.

A few examples are provided below for a better understanding of thepresent invention but should not be considered as limiting the scope ofthe invention itself.

EXAMPLE 1

Using a sensor according to the one described above, and a tincontainer, conductivity measurements are carried out in the presence ofgasoline vapours to which 10% of MTBE has been added.

In FIG. 3.(a) the trace shows the kinetics response of the sensor inrelation to the time at various concentrations of gasoline.

FIG. 3.(b) shows the variation in the resistance in relation to theconcentrations of gasoline. As can be seen the response is proportionalto the concentration logarithm and allows concentrations of less than 1ppm to be detected.

EXAMPLE 2

With the procedure described above, a system consisting of two MTBEsensors and an electronic control unit is prepared.

One of the sensors, protected by an ePFTE membrane, is inserted, up to adepth of about 10 cm, in a tank of 50×40×30 cm full of sandy earth. Thesecond sensor is placed at about 20 cm from the first and about 5 cmfrom the surface.

After a stabilization period of about 30 minutes 1 ml of gasolinecontaining 10% of MTBE is injected with a syringe into the ground, at adistance of 10 cm from the underground sensor and at a depth of 10 cm.In FIG. 4 the temporal point of the injection is indicated with thearrow A. As can be observed, the trace registered by the sensor in theair (2) indicates an almost immediate decrease in resistance, whereasthe trace registered by the sensor in the ground (1) indicates a delayof about 5 min. before the decrease in resistance.

After a few hours, 1 ml of gasoline (indicated with the arrow B in FIG.4B) is injected again. As can be observed, the trace of the sensor inthe ground (1) begins to indicate a decrease in resistance starting fromthe level reached with the previous injection of gasoline. This showsthat the sensor is capable of minitoring a further leakage also startingfrom ground which has already been polluted.

With respect to the trace in air, this starts from a much higherresistance value of the sensor as gasoline vapours dilute very rapidlyin air, unlike the ground where the vapours interstitial tend to remaintrapped.

The time delay of a few minutes in this case, shown by the response ofthe sensor fixed in the ground with respect to that in the air dependson the fact that in the earth interstitial vapours of gasoline and MTBE,although being mobile enough to allow this type of measurement, need acertain amount of time to spread from the leakage point to the sensor.In air the vapours obviously spread at a much faster rate and the sensorconsequently does not show significant delays.

As mentioned in the description, the different behaviour of sensors inthe ground and in the air enables a leakage in the surface to bedistinguished from a leakage in depth in the ground.

1. A process for determining methyl ter butyl ether (MTBE) vapours, inconcentrations equal to or higher than 0.1 ppm, in the ground andoverlying atmosphere comprising: a) adopting a series of MTBE vapoursensors of which at least one in the earth, equipped with a membranepermeable to gases and impermeable to water, and at least one in the airon the surface of the ground, these sensors comprising a sensitiveelement made of a semi-conductor metal oxide containing platinum; and aheater capable of bringing the temperature of said sensitive element toa range of 300 and 500° C.; b) continuously observing the resistancevariations of the sensitive elements by interaction with MTBE, comparingthe signals emitted by the sensor in the earth and the sensor in the airon the ground-surface; and evaluating on the basis of this comparisonthe presence and concentration of MTBE in the surface layers or depthsof the ground and in the atmosphere above the ground itself.
 2. Theprocess according to claim 1, wherein the sensitive element is producedwith tin oxide.
 3. A device for determining methyl ter butyl ether(MTBE) vapours comprising: a) a series of sensors of MTBE vapours eachcomprising a sensitive element produced with a 40 micron layer ofsemiconductor metal oxide containing 1% by weight of platinum, a heatercapable of bringing the temperature of said sensitive element to a rangeof 300 to 500° C., at least one of said sensors being equipped with amembrane permeable to gases and impermeable to water for the protectionof said sensitive element; b) an electronic evaluation system configuredto continuously record the variations in resistance of the sensitiveelements by interaction with MTBE and further configured, to compare thesignals emitted by the sensor in the air on the surface of the ground,and to evaluate on the basis of the compared signals the presence andconcentration of MTBE in the surface layers or depths of the ground andin the atmosphere above the ground itself; wherein the semiconductormetal oxide is tin oxide.